The Alaskan governor, Sarah Palin (centre), and family. Clockwise from rear: son Track, husband Todd, and daughters Bristol, Piper and Willow. Photograph: State of Alaska/Reuters

Sarah Palin, the Republican vice presidential candidate, has revealed that her unmarried 17-year-old daughter is five months pregnant.

A statement released by John McCain's campaign team said Bristol Palin would keep her baby, which is due in late December, and marry the child's father.

"Our beautiful daughter Bristol came to us with news that as parents we knew would make her grow up faster than we had ever planned. We're proud of Bristol's decision to have her baby and even prouder to become grandparents," Sarah and Todd Palin said in the brief statement.

"Bristol and the young man she will marry are going to realise very quickly the difficulties of raising a child, which is why they will have the love and support of our entire family," they added.

The 44-year-old Alaska governor has four other children, the youngest of whom is six months old.

Palin's selection as McCain's running mate was believed to be partly down to her fierce opposition to abortion, in order to placate the Republicans' Christian base, which believes McCain is too liberal on the issue.

Sarah Palin's youngest son, Trig, was born in April with Down's syndrome. Internet bloggers had suggested Trig was actually born to Bristol Palin but that the Alaska governor claimed to be the mother to avoid the controversy. The McCain adviser Mark Salter said today's pregnancy announcement was made to rebut these rumours.

McCain's choice of Palin, the first female governor of Alsaska and the first woman on a Republican presidential ticket, has already drawn controversy, with critics attacking her lack of experience, especially in foreign policy.

McCain's advisers said the Arizona senator had known about Bristol's pregnancy before Palin was asked to become his running mate.